1. Field of the Invention
The present invention relates to a high-sensitivity smoke detecting apparatus for a fire alarm in which a plurality of sample gases are introduced into a smoke chamber in order to calibrate the sensitivity characteristic.
2. Description of the Related Art
When the sensitivity characteristic of a smoke detector is calibrated by introducing a plurality of sample gases into a smoke chamber, two kinds of pure gases such as oxygen gas and freon 12 (F12: CCl.sub.2 F.sub.2) gas, which have greatly different molecular weights, are generally alternately passed through the smoke chamber. The light emitted from a light emitting lamp is scattered by the molecules of the sample gases. The sensitivity characteristic, of the smoke sensor is calibrated by utilizing the phenomenon that the scattered light is captured by a light receiving element.
Namely, since the densities of the pure gases are stable, when a pure gas is introduced into the smoke chamber and is detected as smoke, a very low value of smoke density is output. In other words, the density of the pure gas corresponds to the output low value of smoke density. This can thus be employed for calibrating the sensitivity characteristic of a high-sensitivity smoke detector.
FIG. 7 shows the circuit of a principal portion of a conventional smoke detector.
In this conventional example, the light emitted from a xenon lamp (not shown) is scattered by smoke, and the scattered light is received by a light receiving element 81. The output signal from the light receiving element 81 is amplified by an amplifier 82, and the peak value thereof is held by a peak holding circuit 84. The data obtained by subtracting a predetermined value from the peak value using a subtraction circuit 85 is transmitted to a receiver through a signal output circuit 87. A gain adjusting variable resistor 83 adjusts the gain of the amplifier 82, and an offset adjusting variable resistor 86 adjusts the predetermined value subtracted from the peak value.
FIG. 8 shows the values of current I output from the signal output circuit 87 relative to the smoke density (gas density) when pure oxygen gas and freon 12 gas are alternately introduced into the smoke chamber. In FIG. 8, line C1 shows the characteristics when the sensitivity of the smoke detector is not adjusted, and line C3 shows the characteristics when the sensitivity has been adjusted. On the line C3 obtained by adjusting the gain adjusting variable resistor 83 and the offset adjusting variable resistor 86, the output currents of the signal output circuit 87 when oxygen gas is used and when freon 12 gas is used are denoted by I1 and I2, respectively.
Such adjustment of the sensitivity of the smoke detector is performed by regulating the slope of the line C1 and moving the line C1 in parallel. The output of the smoke detector is, in fact, adjusted while an ammeter or the like is observed. The freon 12 gas is first introduced into the smoke chamber, and the slope of the characteristic line C1 is adjusted by operating the gain adjusting variable resistor 83 so that the output I of the signal output circuit 87 may become I2 to obtain the characteristics shown by a line C21. The oxygen gas is then introduced, and the line C21 is moved parallel so that the output I of the signal output circuit 87 may become I1 by operating the offset adjusting variable resistor 86 to obtain a line C22. The freon 12 gas is again introduced, and the slope of the line C22 is adjusted by operating the gain adjusting variable resistor 83 so that the output I may become I2 to obtain a line C23. The above operation is repeated until the characteristic line C3 is obtained.
In the change from the characteristic line C1 to the characteristic line C3, when the offset adjusting variable resistor 86 is adjusted (parallel movement) so that the output data at the time of use of oxygen gas may become the value I1, the output value corresponding to the freon 12 gas deviates from the value I2, and thus the characteristic line C3 cannot be obtained. When the gain adjusting variable resistor 83 is then adjusted (adjustment of slope) so that the output data at the time of use of freon 12 gas may become the value I2, the output value corresponding to oxygen gas deviates from the value I1, and thus the characteristic line C3 cannot correctly be obtained. Therefore, the sensitivity adjustment cannot be completed by measuring the characteristics using oxygen gas and freon 12 gas only once.
Although the characteristics, of course, gradually approach the characteristic line C3 in the process of repeating the above measurements, several measurements of the characteristics are required. Since the gases used must be completely replaced by new gases at each measurement, the gas replacing work is difficult and troublesome. The above conventional smoke detecting apparatus thus has the problem that the work of adjusting the sensitivity of the smoke detector is troublesome and requires much time.